Scheelite and wolframite concentrates are processed to produce ammonium paratungstate (APT), a precursor for the production of commercially pure tungsten and tungsten compounds. As an initial step, metal values in the concentrates are solubilized by alkaline digestion with sodium hydroxide or sodium carbonate to form sodium tungstate solutions. Caustic digestion may be accomplished at atmospheric or superatmospheric pressure and at elevated temperature depending upon the nature of the feed concentrate. In one commercial operation, the solution resulting from alkaline dissolution of the feed ore was reported to contain 50 to 300 gpl tungsten, about 10 to 10,000 ppm molybdenum and metal impurities such as lead, copper, antimony, bismuth, zinc and arsenic, all of which can be wholly or partly co-precipitated with molybdenum trisulfide. Separation of molybdenum from such a solution by sulfide precipitation accordingly will give a precipitate which is quite small in volume as compared to the volume of the original ore.
It is known from U.S. Pat. No. 2,339,888 that tungsten and molybdenum values contained in a scheelite concentrate can be dissolved in aqueous sodium hydroxide at 130.degree. C. to form a solution containing sodium tungstate and sodium molybdate and that molybdenum sulfide could be precipitated from the solution by adding sodium sulfide, heating to 70.degree. C. and adjusting the solution pH to a pH between 3 and 5 with hydrochloric acid. U.S. Pat. No. 4,303,622 addresses the problem of treating sodium tungstate solution high in molybdenum such as might be generated from caustic digestion of low-grade wolframite sources. The process disclosed utilizes a two-stage sulfide precipitation with intermediate caustic dissolution of the initial molybdenum sulfide cake. U.S. Pat. No. 4,303,623 is directed to precipitation of molybdenum trisulfide from sodium tungstate solutions having a pH between about 8 and about 5 by adding sulfide in excess of the amount required to form sulfides with molybdenum and other sulfidizable species present and then rapidly acidifying the solution through the pH range of 5 to 4.5 to a final pH of 2.5. A tungsten solution very low in molybdenum is obtained.
Despite the multitudinous relevant teachings in the art as exemplified by the foregoing, it is still found that, in attempting to operate a commercial plant designed to treat concentrates containing mixed tungsten-molybdenum values, losses of tungsten to the molybdenum trisulfide cake were excessive. It was foreseen, also, that such tungsten losses would become even more severe as the grade of tungsten concentrate changed in the direction of higher molybdenum contents. Even when molybdenum contents in the caustic leach liquor are only 0.02 to 0.05 gpl, 1% to 2% of tungsten in the solution can be lost in the molybdenum trisulfide cake.
The tungsten values in the molybdenum trisulfide cakes cannot readily be recovered by direct recycling of the cakes back into the plant process, since these cakes represent the only method for removing molybdenum values from the feed to the plant. If the cakes are recycled for tungsten recovery without first removing all or most of the molybdenum, molybdenum levels in process solutions will increase steadily, and ultimately will lower ammonium paratungstate (APT) product quality. In the past these cakes have been treated several ways, but the two most common methods have been either to roast the trisulfide cake to produce a low grade oxide containing both tungsten and molybdenum for sale to the steel industry, or to dispose of the cake in a landfill. In the latter case, the tungsten values are completely lost, while in the former case the tungsten is sold for significantly less than if it were sold as APT.
It becomes apparent therefor, that an improved treatment scheme is required in order to recover tungsten from precipitates. It is to the provision of such a treatment scheme that the present invention is directed.